Researchers grow self-aligned nanotubes

PORTLAND, Ore.  Growing carbon nanotubes on silicon could enable nanoscale transistors, but only if designers can specify exactly where the tiny devices grow. Engineers at Case Western Reserve University have demonstrated how to grow nanotubes exactly where designers want them  self-aligned across a wafer.

The researchers also reported that the nanotubes self-welded during growth, and claimed that their nanotube-enabled wafers need only to be diced and wire-bonded to a chip carrier. This would make them as reliable and cost effective as current chips.

"Our contribution has been to come up with a technique where the nanotubes grow where you want them to grow  between two electrical posts," said Massood Tabib-Azar, a professor in the electrical engineering department at Case Western (Cleveland).

Others have experimented with nanotube-based transistors, but have usually studied them where they happened to be located on a substrate. Some experiments have harnessed electric fields to grow nanotubes in selected locations, but that technique is not scalable across a wafer.

Case Western's method, on the other hand, aims to locate nanotubes where they are required on a chip and scale the process across a wafer so nanotubes grow simultaneously and weld themselves in place.

"The self-welding is the really critical breakthrough," said Tabib-Azar. "They grow from one post to the other post, and when they reach the other post they weld themselves to it."

Next, the researchers plan to build an ultrasensitive nanotube-based sensor chip for detecting gases. The multi-walled carbon nanotubes being grown exhibit p-type conductivity, and the current through them reduces when exposed to ammonia or hydrochloric gases, making a gas sensor chip a logical first application.